The human species has co-evolved over hundreds of millennia with an array of commensal microbes, a symbiotic relationship now altered by profound changes in lifestyle, hygiene, or widespread antibiotic use, changes that may be linked to the increasing incidence of allergy or autoimmune diseases. Intestinal microbes strongly influence immune cell differentiation, as exemplified by Segmented Filamentous Bacteria (SFB), a gut- resident anaerobe which promotes the differentiation of IL17-producing Th17 cells, which in turn unleash Th17-dependent autoimmune diseases. This project will analyze the mechanisms of the SFB>Th17 axis, a well-defined and tractable model system of the microbial influence on immune function. Gene expression profiling of changes induced by SFB in the small intestine brought forth several intriguing candidates, and we have obtained formal evidence that one of them, the serum amyloid protein SAA3, constitutes an essential relay in the cascade. We will build on these results to analyze the interplay between SFB and Th17 differentiation, and how the effects of SFB are influenced and integrated within other microbial or environmental influences. 1. Identify the sequence of cells and molecules through which SFB induces Th17 differentiation in vivo. We hypothesize that SFB elicits a response cascade, initiated in the epithelial cells with which SFB interacts, transmitted via macrophages of dendriti cells, which ultimately promotes the Th17 differentiation of CD4+ ??T cells. Genomic profiling show that SFB induces the expression of a number of genes whose products may function as intermediates in the cascade: the serum amyloid SAA3 whose importance we have begun to establish, and other candidates (SAA1, iNOS, Resistin-?). We will chart the sequence of events: (i) identify the cell-types in which these putative intermediates are induced; (ii) assess with inducible transgenesis and knockout mice which segment of the response maps downstream of each intermediate. 2. How SAA3 induces the Th17 in ??T cells. Recombinant SAA3 can induce Th17 differentiation but only in the presence of myeloid CD11b+ od CD11c+ cells. We will establish the mechanism at play by establishing which myeloid cell(s) can serve as the essential cofactor, analyzing which signaling pathways are triggered by SAA3, and testing which intermediate cytokines may be induced and required. 3. Are SFB effects modulated by, other microbes or environmental cofactors. It is likely that SFB's influence is modulated by other gut commensals, or by dietary factors. We will test this notion by correlating the micro- heterogeneity in SFB's effects with the abundance of other bacterial families, determined by microbial community genetic profiling, and by testing how SFB-induced Th17 cells and arthritis manifestations are influenced by dietary elements (Ahr ligands, NaCl). These studies should provide novel mechanistic insights into how the gut microbiota alters the host's immune system and modulate the risk of autoimmune diseases, mechanistic information which may suggest new strategies for addressing human autoimmune disease.